In 1971, David Bowie sang a question that has become only more relevant in the 45 years since the release of Hunky Dory: is there life on Mars? Several recent scientific discoveries have elevated the pondering of this question from the lowly lands of fantastical thinking to the high plains of Serious Scientific Endeavors. In this installment of the ongoing “Life, uh, finds a way” series we will attempt to place these discoveries in the context of chemical ecology and, in the process, remind everyone that yes, space is cool.
For many years, it's been known that there is plenty of ice water on Mars. The poles of Mars are covered with ice caps that are a mixture of frozen water and carbon dioxide (aka dry ice, the stuff you used in high school to lend low budget theater productions a creepy mood). However, life on Earth requires water in the liquid phase: even microbes that live in glaciers are only metabolically active when and where there is liquid water in their environment. Specifically, the bacteria that inhabit Antarctic glaciers actually live within small (microns wide) veins of liquid water that form in the ice where the local salt concentration is very high. This high salt concentration decreases the freezing point of the water to a degree such that, even at temperatures much below freezing, liquid water can still flow. The microbes that are capable of living in these glacier veins with very high salt concentrations are known as halophiles, meaning “salt-loving.” Bacteria that have adapted to environments such as the veins of glaciers can handle the osmotic stress that is placed on the cell membrane in high-salt concentrations- conditions that would likely kill a bacterium that lives in a fresh-water lake, for example.
So microbiologists and origin-of-life scientists alike were extremely excited when NASA announced in September of 2015 that they'd discovered clear evidence for the presence of liquid water on Mars. All of Mars' known liquid water is localized to features named “recurring slope lineae” (RSL): dark, 100-meter long streaks found in multiple locations across the planet that appear in the warmer months (when the temperature exceeds -10°F) and disappear in colder months. The RSL are composed of liquid brines, aqueous solutions of salts including magnesium perchlorate and sodium perchlorate. Much like the liquid water veins that can form in Arctic glaciers, liquid water can only form on Mars due to the presence of high concentrations of salts. But you may be asking, why perchlorate? Perchlorate is a highly abundant compound on Mars (it makes up about 0.6% of the weight of Martian soils), and is hygroscopic, meaning it readily binds to water to form hydrates.
The abundant perchlorate on Mars is what makes liquid water possible on the planet. But could anything live in this perchlorate brine?
To help answer this question, let's examine the state of perchlorate on our planet. Perchlorate is not a naturally abundant compound on Earth, which is good for us because it is fairly toxic to humans. Perchlorate blocks iodide uptake by the thyroid, which can lead to a host of thyroid-related metabolic issues. Despite its toxicity, there is a big demand for man-made perchlorate salts. Ammonium perchlorate is used as an oxidizer in modern day rocket fuels, which make such explosive things like spacecraft and expulsion seats possible. While some perchlorate ends up in fireworks and pesticides, about 90% of manufactured perchlorate is destined for aerospace and military applications. Unfortunately, every time the military updates or retires rockets, massive amounts of perchlorate are disposed, increasing the chances of its release into the environment. Perchlorate is highly water soluble and is thus easily dispersed in environmental waterways (rivers, streams, groundwater etc). It is also a very stable chemical, so once in the environment it tends to stick around. To help protect people from perchlorate toxicity, the Environmental Protection Agency (EPA) has set the limit of the allowable concentration of perchlorate in tap water to 6 parts per billion, which is approximately six orders of magnitude less concentrated than the perchlorate levels in Martian soil.
The interesting part of the perchlorate story is how we've decided to clean it up. The most efficient way to detoxify perchlorate-contaminated water is through a process called bioremediation, defined by the EPA as “a treatment process that uses naturally occurring microorganisms (yeast, fungi, or bacteria) to break down, or degrade, hazardous substances into less toxic or nontoxic substances.” It has been known for about fifty years that naturally occurring bacteria can degrade perchlorate, and ill-defined microbial communities that include these perchlorate-reducing bacteria have been used to treat contaminated water successfully. It's only been in the past fifteen years or so that the biology of perchlorate-reducing bacteria has been studied in any detail. The results of these studies have revealed that some strains of bacteria can actually grow through the reduction of perchlorate to generate ATP (the primary energy source of cells). The reduction of perchlorate results in the production of two harmless end products, oxygen and chloride. Interestingly, perchlorate-reducing strains only perform this chemistry under anoxic (oxygen free) conditions, as these bacteria preferentially utilize oxygen over perchlorate for energy production.
While various strains of perchlorate-reducing bacteria have been isolated from anoxic water-treatment plants contaminated with perchlorate salts, others have been found in pristine environments. This raises the important question as to why these unrelated bacteria found in diverse environments have evolved the ability to reduce perchlorate. You may recall from our previous installment of the “Life, uh, finds a way” series that Deinococcus radiodurans (affectionately named rad) was able to withstand crazy high levels of ionizing radiation. After much wild speculation as to why rad had evolved to withstand radiation that is not found in any natural habitat on Earth, scientists discovered that rad's resistance to radiation was a side effect of it's adaptation to survive dessication, an ability that fits in with the needs of its known terrestrial lifestyle. Given the paucity of perchlorate on Earth, and the fact that perchlorate-reducing bacteria are not closely related in terms of genetics or habitat, it's likely that perchlorate-reduction has arisen from adaptation to some other requirement. For instance, based on the enzymes implicated in the reduction of perchlorate, biochemists studying the issue have posited that perchlorate reduction has arisen from the evolution of these strains to reduce nitrate, a common oxidant on Earth.
To review: First, there is liquid water on Mars that isn't pure water, but rather a brine that contains high levels of perchlorate salts, which are toxic to humans. Second, there are bacteria on Earth that can reduce perchlorate and even use perchlorate to grow. Given these pieces of information, I think it's not so crazy to think that, if the only hurdle to life on Mars was the requirement to utilize perchlorate for energy production, there could be or could have been life in the RSL. However, since we are pondering these issues while pacing back and forth in the leather armchair and pipe smoke-filled land of Serious Scientific Endeavors, there are a few more issues we should consider when we ask the question, is there life on Mars?
Let's consider one such hurdle to life on the red planet. Mars lacks a global magnetic field, meaning that solar wind (high energy, charged particles released by our Sun) has been bombarding the planet for about 4 billion years, making a protective atmosphere like that of Earth impossible. As a result, if you were to spend a year on Mars, you'd receive about 500-times more ionizing radiation than if you'd stayed on your couch reading this article (hopefully it doesn't take you a year to read this, but hey not judging). The constant onslaught of ionizing radiation on the surface of Mars would kill off even the amazing rad, because the bonds found in organic molecules are simply not sturdy enough to withstand these energy levels over long periods of time.
Finally, let's turn to a big issue that needs to be discussed whenever we start theorizing about the possibility of life outside of our beautiful green world. While we mostly understand the basics of what is required for life on Earth, that understanding only applies to the version of life that appears on our planet, which happens to be the only version of life we know. If we are searching for life on other planets and we think that it should take the form of life we have on Earth, than we can clearly see if the checklist for life (which includes, among other things, liquid water, lack of ionizing radiation that breaks apart organic molecules, and a supply of elements with which to build cellular structures) is satisfied on that planet. However, at least in my mind, I really don't believe for one second our version of life, which comes down to DNA inside of a membrane, is the only version out there. And when we start thinking that life on another planet may take another form altogether, it becomes pretty much impossible to even make that “need for life” checklist, let alone check it.
So, at least for now, I think that we can pretty safely conclude that, no, there is most likely no life as we know it on Mars right now. But there might be life (read: humans) on Mars in the future, and it is useful to think about what elements of the Martian environment need to be changed to make the planet habitable for us weaklings. For instance, could we send engineered bacteria to Mars to clean up the soil and make it habitable for us? Perhaps we could use perchlorate-reducing bacteria to detoxify the soil and generate oxygen for our breathing needs in the process. Ultimately, our study of the strange ways of microbes on Earth could impact future space exploration and may enable us to colonize another planet. If that planet is Mars, there will definitely be ample fireworks supplies. So, Happy Mars Day! Cue explosion.